Effects of Processing Time, Mixing Speed, and Mixer on

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Effects of Processing Time, Mixing Speed, and Mixer on Agglomerates in Fuel Cell Cathode Inks

Erin B. Creel (ORNL)

Carlos Baez-Cotto (NREL), James Young (NREL), Scott A. Mauger (NREL), Michael Ulsh (NREL), David L. Wood III (ORNL), and Alexey Serov (ORNL)

22

Large Agglomerates Reduce PEM Fuel Cell Performance

ACS Appl. Energy Mater. 2019, 2 (9), 6417–6427.

Aggregates (smaller)

Agglomerates (larger)

1 min. bath sonication:

• More agglomerates

• Worse performance

10 s tip + 20 min. bath sonication:

• Fewer agglomerates

• Improved performanceBath Sonication Tip Sonication

33

Large Agglomerates Reduce PEM Fuel Cell PerformanceAggregates

(smaller)Agglomerates

(larger)

1 min. bath sonication:

• More agglomerates

• Worse performance

10 s tip + 20 min. bath sonication:

• Fewer agglomerates

• Improved performance

ACS Appl. Energy Mater. 2019, 2 (9), 6417–6427.

44

Pt/C catalyst, Nafion ionomer,

water, and alcohol shear

mixed

Shear viscosity of mixed ink measured

Ink coated on GDL with wire-

wound rod and dried at 80 °C

Agglomerates found, filtered,

and highlighted in micrographs

of GDE

Methods for understanding mixing of fuel cell inks

Mixing RheologyGDE

FabricationParticle Analysis

Continue mixing

55

Optical images at 500X magnification over mixing time 5 minutes 10 minutes 15 minutes

20 minutes 25 minutes 30 minutes

IKA 18G, 10,000 rpm

66

5 minutes 10 minutes 15 minutes

20 minutes 25 minutes 30 minutes

Optical images at 500X magnification over mixing time IKA 18G, 10,000 rpm

77

Effect of rotor-stator mixer speed on large agglomerates

• Large agglomerates in micrograph area counted

• Number of large agglomerates decays exponentially with mixing time

• All speeds achieve an “ultimate fineness”

• Higher speeds break upmore agglomerates

18G

88

Effect of rotor-stator geometry on particle sizes

• Rotor-stator geometry has a greater impact on number of large agglomerates than rotational speed

Mixer

Model

Dispersing

Element

Rotor/Stator

Gap# Rotor Teeth

Rotor

DiameterShear Number

Rotor Tip

Speed

18G 0.3 μm 2 12.7 mm3 × 108

@ 20,000 rpm

0.013 m/s

@ 20,000 rpm

25F 0.5 μm 8 18.0 mm1 × 1014

@ 7,000 rpm

0.007 m/s

@16,600 rpm

99

Effect of rotor-stator geometry on particle sizes

• Shear number

– = shear rate × shear frequency

– Not sufficient in predicting the number of large agglomerates

• Rotor tip speed is a better predictor of number of agglomerates

Mixer

Model

Dispersing

Element

Rotor/Stator

Gap# Rotor Teeth

Rotor

DiameterShear Number

Rotor Tip

Speed

18G 0.3 μm 2 12.7 mm3 × 108

@ 20,000 rpm

0.013 m/s

@ 20,000 rpm

25F 0.5 μm 8 18.0 mm1 × 1014

@ 7,000 rpm

0.007 m/s

@7,000 rpm

0.013 m/s0.016 m/s

0.007 m/s0.007 m/sRotor tip speeds:

1010

Ink property comparison between mixer types

Mixing time (h)

Shear

Vis

cosity (

Pa.s

.)

Mixing time (min)

Shear

Vis

cosity (

Pa.s

.)Mini-Homogenizer (6k rpm)

Ball Mill (50 rpm)

Viscosity at a fixed 1 /s shear rate

Quickly achieves ultimate fineness

• Long mixing timesneeded

• Slowest motor speed but best agglomerate break up

Ultimate shear viscosity = 1 Pa.s

Ultimate shear viscosity = 1 Pa.s

Mixing time (min)

Shear

Vis

cosity (

Pa.s

.)

IKA 18G (10k rpm)

• Highest motor speed but worst agglomerate break up

Ultimate shear viscosity = 1 Pa.s

1111

Future Work

• Fuel cell cathode performance evaluation and correlation with agglomerates

– Does fewer larger agglomerates result in improved performance?

– Does excessive mixing damage the ink and decrease performance?

• Explore other models to better predict number of large agglomerates per cathode layer area

Questions? Attend the live Q&A or email

creeleb@ornl.gov